Injection of various gases into an oil reservoir in order to enhance the oil recovery from the reservoir, and to stabilize it, has for a long time been known and used. Gases such as CO2, N2 and natural gas will reduce the surface tension between gas and oil, and thus contribute to both increased recovery and stabilization of the reservoir.
During enhanced oil recovery operations, a number of techniques are applied that depend on the nature of the specific field and wells, their maturity, seasonal variations etc. The most common approaches are secondary oil depletion using water flooding or gas injection. Further alternatives, often referred to as tertiary depletion, include injection of gas after water, alternating gas and water injection (WAG), and simultaneous water and gas injection (SWAG). Thermal treatment by injection of steam or in situ combustion is also possible. By gas we here mean all viable options like methane, other hydrocarbons, nitrogen, air, flue gas, carbon dioxide or mixtures of any of these gases.
Natural gas as such may be injected into fields where the gas does not have a net value that exceeds the excess profits of increasing the oil recovery in the field. An oil field contains hydrocarbon liquids (oil), associated gas and water.
N2 may be produced together with O2 in a so-called air separation unit (ASU). In an oil field, such an air separation unit will normally produce N2 with a purity of >99.9% and oxygen-enriched air. There is little or no need for this oxygen-enriched air on the oil field, and all or most of this is therefore released.
Separation of air into an “oxygen-depleted stream” and an “oxygen-enriched stream” is described in U.S. Pat. Nos. 5,388,645 and 6,119,778. The oxygen-depleted stream is used for injection into a “solid carbonaceous formation” for improved recovery of methane, and at least a part of the oxygen-enriched stream is used for reaction with a reactant stream containing at least one oxidizable reactant. Examples of processes are steelmaking operations, production of non-ferrous metals, chemical oxidation processes and production of synthesis gas for Fischer-Tropsch synthesis of higher hydrocarbons from natural gas.
The oxygen-depleted stream has a nitrogen to oxygen volume ratio of 9:1 to 99:1. A too low ratio may lead to the formation of an explosive gas. An oxygen-depleted gas, e.g. nitrogen, for injection into an oil field to enhance the production preferably includes less than 0.1% oxygen.
No other integration between the processes using the oxygen-depleted and oxygen-enriched streams is mentioned in U.S. Pat. Nos. 5,388,645 or 6,119,778.
Natural gas may also be used as feed for a number of processes such as the production of methanol, di-methyl ether or other oxygenated hydrocarbons, and/or synthetic fuel/propellant. This can take place in accordance with known processes such as described in PCT/NO00/00404.
The oxygen needed for production of synthesis gas (“syngas”) in plants for synthesis of methanol and other oxygenated hydrocarbons and/or synthetic fuel may be supplied as straight air containing about 21% O2, oxygenated air containing more than 21% O2 to pure oxygen of almost 100%.
Syngas is a mixture of CO, CO2, H2, N2 and water vapour and some non-reacted natural gas. The syngas is used in various synthesis reactions, such as for the production of methanol and other oxygenated hydrocarbons, heavier hydrocarbons and ammonia. The N2 in the syngas enters the system as a part of the natural gas and/or air or oxygenated air. Nitrogen is inert both in the syngas production and in the different following synthesis reactions and finds its way into the tailgas stream which is then passed through a tailgas combustion unit to produce power. It is often preferred to use oxygenated air or pure oxygen for the production of syngas to reduce the total gas volume of the gas circulating in the plant.
The oxygen produced in an air separation unit in such a plant is typically >95% pure oxygen, while the nitrogen often will be relatively impure that is not suitable for other applications, and is therefore released to the atmosphere.
A process for preparation of higher hydrocarbons and for enhancing the production of crude oil from an underground formation is described in Canadian Patent No. 1,250,863. The off-gas from the synthesis plant is oxidized into mainly CO2 and H2O before it is injected into the underground formation. Preferably, the presence of nitrogen is avoided by using oxygen from an air separation unit for all oxygen-demanding processes.